New processes are continually being sought to increase the recovery of oil. Reservoirs in the U.S. have produced more than ten billion barrels of oil and are estimated to contain over 90 billion barrels in place. If deposits which have little or no production history were included, the total remaining oil is estimated to be in excess of 150 billion barrels. Total oil recovery cannot be realized either in natural water drives or secondary recovery waterflooding operations. This condition is because water films bridge the gaps between sand particles, or close off the pore channels in the formations, after trapping the oil. At this time there is no way to tap the potential recovery discussed. The most difficult task anticipated is the recovery of heavy crudes. The incentive for developing a process that will include high viscosities in its capabilities is easy to illustrate. There are more than 1500 fields in 20 States containing heavy crudes alone. The present invention is directed to a system that will enhance production of all viscosities of crudes.
The adhesion of oil and water to surfaces of sandstone impedes the flow. Sometimes oil exists in reservoirs as individual droplets. Hence, there is little or no reservoir pressure which could be used to force the oil toward a well in the formation. Also, the oil is frequently so viscous that it has few of the characteristics that a completion engineer would consider desirable. It is desirable therefore to provide a dual force that will separate the pockets of crude from their individual places and force the resulting free material to a well bore. This dual force must be capable of propagating effective energy over an extensive subsurface zone in order that the entire area of the oil bearing formation may be produced. Further, the energy must be capable of being brought to the formation through an eight-inch well bore thousands of feet deep. For collection, the oil must be forced through the formation by the energy. A suitable technique for applying energy that alters the distribution of fluids in a formation influences the flow characteristics and hence improves recovery. It is to this end that the present invention finds its direction.
Since a large percentage of the oil remains in the reservoir after primary and secondary treatment, even relatively small recovery would be desirable. If only an additional 10 to 15 percent could be removed, it is obvious that the profit resulting would be exceptional. Calculations in accordance with the present method indicate that as much as 70 to 80 percent of the oil can be brought to well bores for production. This estimate is for most primary reservoirs that may be lacking reservoir energy. Under such conditions what was initially considered as marginal production with conventional techniques can prove to be a lucrative effort. Of course, porosity and permeability must be present along with saturation. But indications are that low permeability can be improved substantially by removing blockage, whether it be water or cementing between sand grains of the formation. This fact means that cementing material can be broken down and even fractured by an appropriate shock front, thereby increasing permeability. In accordance with the present invention, an infinite selection of shock waves and their energy can be made and adjusted to the particular subsurface condition. In this way preliminary treatment can be followed by oil movement.
In appraising a potential operation it is necessary to make simplifying assumptions because of the complicated nature of the many factors affecting performance. For example, consider the Buckrange reservoir, Stephens Field, Arkansas. Over 100 million barrels of oil remain in the reservoir that cannot be recovered by primary producing methods. The reservoir is about 2100 feet deep and its structure is a low dipping, southeast-plunging nose or terrace. The Buckrange sandstone (Upper Cretaceous) is terminated on the north and northeast sides by a fault and shales out in the other directions. The sandstone is fine grained and contains varying amounts of shale. Porosity, permeability and water saturation are variable. The average net porosity is 31 percent, the average net permeability is 92 millidarcys, and the average interstitial water saturation is 43 percent. The volume of stock-tank oil initially in the reservoir was estimated to be 126 million barrels, or 1,284 barrels per acre foot. Primarily, production has amounted to only 94 barrels per acre foot on a field-wide basis since its discovery in 1922. Theoretical calculations on a water flood indicate that a maximum of 278 barrels of flood oil per acre foot may be recovered by an efficient flood. Actual recovery, however, should be less than this. In this type reservoir the shock wave technique of this invention is calculated to produce approximately 900 barrels per acre foot. This is about 75 percent additional oil production after primary as compared to an estimated 23 percent additional recovery from an efficient water flood. In theoretical calculations, such as were used, the permeability-saturation relationships, saturation gradient, geometry of the system, etc. affect the time element in estimating recovery. In linear flow the geometry is less complex than in radial flow. Also, when deviation from the true radial characteristic is considered, the complexity is compounded. As more variables are included, computer time becomes essential.
In order for a secondary recovery technique to be desirable, certain economic guidelines must necessarily be considered. It is desirable that present wells be utilized to gain access to the oil bearing formation by energy generating apparatus. The apparatus therefore should be capable of passing through a conventional drill collar to eliminate any need for expensive enlargement of the well bore. The process should be economical in comparison with prices in the crude market, should develop the equivalent of thousands of horsepower, and be so controlled that it is non-destructive. The energy source should be inexpensive and should be reusable many times in order to enhance the commercial aspects of the recovery method.